Electric field-effect devices, such as field-effect transistors (FET), with semiconducting current channels between drain and source electrodes are well known and widely used in many applications. After the discovery of the high-T.sub.c superconductive materials, European published patent application number EP-A-0 324 044 published 19 Jul. 1989 disclosed that high-T.sub.c superconductive materials may bear an electric field effect which is much larger than that in low-T.sub.c superconductive materials. The length scale by which electrostatic fields are screened in conducting materials is generally given by the sum L.sub.D +L.sub.DZ of the Debye length L.sub.D =(.epsilon..sub.o .epsilon..sub.r kT/q.sup.2 n).sup.1/2 and the width of eventual depletion zones L.sub.DZ =N/n. Here, .epsilon..sub.o and .epsilon..sub.r are the dielectric constants of the vacuum and of the conducting material, respectively, k is Boltzmann's constant, T is the absolute temperature, q is the elementary charge, n is the density of mobile carriers, and N the induced areal carrier density. Because of their high carrier density, low-T.sub.c superconductors usually screen electric fields so well that the fields only have a minor influence on materials properties. To attenuate the screening, recent experiments on the electric field effect in low-T.sub.c superconductors have focused on compounds with exceptionally low carrier density, like doped SrTiO.sub.3, with niobium as the dopant, for example ( cf. European published patent application No. EP-A-494,580, published 15 Jul. 1992).
As described in European published application No. EP-A-0 324 044, field-effect devices with superconducting drain-source channels would offer advantages over their semiconducting counterparts. In high-T.sub.c superconductive compounds, larger field-effects can occur owing to their intrinsically low carrier concentration, and because of their small coherence length. The low carrier concentration of roughly 3 to 5.times.10.sup.21 /cm.sup.3 leads to screening lengths in the range of tenths of nanometers, and the small coherence lengths allow the fabrication of ultrathin layers with respectable critical temperatures. Superconducting films as thin as 1 to 2 nm have been grown; electric fields can penetrate such films to a considerable extent. Various types of FETs with superconducting channels have already been suggested, e.g. in European published patent application No. EP-A-0 324 044; in European published patent application No. EP-A-0 494 580 (application No. 91 810 006.6); in D. F. Moore, "Superconducting Thin Films for Device Applications," Second Workshop on High Temperature Superconducting Electron Devices, Jun. 7-9, 1989, Shikabe, Hokaido, Japan, pp. 281-284; and in J. Mannhart et al., "Electric field effect on superconducting YBa.sub.2 Cu.sub.3 O.sub.7-.delta. films" Z. Phys. B--Condensed Matter vol. 83, pp. 307-311 (1991).
However, the devices of the citations of the previous paragraph generally require either an operating temperature close to a critical temperature T.sub.c , as measured effectively approaching the limit of zero current and zero applied magnetic field, or channel layers which are extremely thin.
It is an object of the present invention to provide superconducting field-effect devices where the operation is influenced by the control of critical current and flux flow by an electric field.
It is a further object of the invention to provide for the operation of superconducting field-effect devices at any temperature from T=0 to T.congruent.T.sub.c. (It has been observed that the effect is present even at temperatures slightly above the critical temperature T.sub.c as measured in an effectively zero electric field, provided the electric field acts to increase T.sub.c).
It is a still further object to provide superconducting field-effect devices whose conducting channel (between drain and source in case of a FET, for example,) may be grown thicker than permitted in conventional prior art devices.